There are numerous circumstances in connection with the completion of wells and/or the production of hydrocarbons, i.e. oil or gas in which it is desired to place a thickened aqueous composition, that is to say a fluid or gel which is more viscous than water, at a location in a subterranean wellbore or in a subterranean geological formation.
Such viscosified fluids may be used for a variety of purposes including for example fracturing to stimulate production, reducing permeability where it is not required, zonal isolation and fluid drive from an injection well towards a production well.
The literature contains many proposals for the formulation and also for the placing of such thickened compositions. The viscosity required for a thickened composition will vary according to its intended function, and so for example fluids for zonal isolation may be very viscous gels intended to remain immobile whereas fluids for other functions may be required to be somewhat less viscous so that they can be moved by pump pressure. Nevertheless, there are obstacles to overcome. Pumping a viscous composition from surface to a subterranean location requires a great deal of energy and limits the viscosity of the composition which can be delivered downhole. Even formulating a very viscous composition at the surface is difficult. Methods for increasing viscosity when the composition is already underground have been proposed, but may involve cost or hazard associated with chemicals used and maybe applicable in only a limited range of circumstances.